24C02 EEPROM (24LC02B, AT24C02): pinout, I2C address and pull-ups

"24C02" is a generic name. Shops sell Microchip's 24LC02B and 24AA02, the Atmel AT24C02 and its later C and D versions, and unbranded copies under it. The pins, the 1010 control code, the 8-byte page and the 5 ms write time are the same on both sheets read for this page; the A0 to A2 pins are not.

The 24LC02B runs from 2.5 V to 5.5 V; the 24AA02 on the same sheet goes down to 1.7 V but drops to 100 kHz below 2.5 V. The AT24C02-2.7 runs from 2.7 V to 5.5 V. Atmel now marks the original AT24C02 not recommended for new designs, so new stock carries a later letter.

The first byte of every transfer is the control byte: 1010, three more bits, then R/W. On the 24LC02B the three middle bits are "don't care" and the A pins have no internal connection, so the part answers at 0x50 whatever you wire to them. On the AT24C02 those bits must match the A2, A1 and A0 pins, so tying all three to ground gives 0x50 and other combinations give 0x51 to 0x57.

Both parts run from a 5 V or a 3.3 V board. The 24LC02B takes 6.5 V on VCC before damage, and no input may go more than 1.0 V above VCC, so a 5 V signal must not reach a 24LC02B powered from 3.3 V.

A write is not finished when the Stop bit goes out. The chip then spends up to 5 ms programming the cells and ignores the bus while it does. Writing a page of 8 bytes costs the same 5 ms as writing one byte, so filling all 256 bytes takes 32 page writes, 160 ms, against 1.28 s one byte at a time.

A page write that runs past the end of its 8-byte page wraps round to the start of the same page and overwrites it. Pages start at addresses that are multiples of 8, so split writes on those boundaries.

SDA is open drain: the chip can only pull it low, and a resistor to VCC brings it back up. The resistor has two limits. Too small, and the chip cannot pull the line under 0.4 V: at 3 mA that sets the floor at (5 V − 0.4 V) / 3 mA = 1.53 kΩ on 5 V, 967 Ω on 3.3 V. Too large, and the line rises too slowly. The 24LC02B sheet allows 300 ns from 30 % to 70 % of VCC, and an RC edge takes 0.8473 × R × C to do that. With 100 pF of bus capacitance the ceiling is 3.54 kΩ.

2.2 kΩ sits in the middle: a 186 ns rise, and 2.09 mA for the chip to sink on 5 V (1.32 mA on 3.3 V). Microchip's pin section suggests 10 kΩ for 100 kHz and 2 kΩ for 400 kHz. On 100 pF, 10 kΩ gives 847 ns, which meets the 300 ns figure only on a bus under 35.4 pF: one chip and short wires. Count the pull-ups already on the bus, too. Many sensor modules carry their own, and they add in parallel.

SCL gets the same resistor. The pull-up and pull-down resistors lesson covers the general case.

• After a write, poll for the acknowledge instead of waiting a fixed 5 ms: send Start and the write control byte until the chip answers. Microchip's sheet describes this as acknowledge polling.
• Tie WP to ground to write. Tied to VCC, the whole array is read-only; reads still work.
• An I2C scanner that finds nothing at 0x50 usually means missing pull-ups or SDA and SCL swapped, not a dead chip.
• Fit 100 nF from VCC to ground at the package.
• Do not write in a loop without a delay or a change check. It is the fastest way to use up the million cycles.

Any 24C02 or 24LC02B drops into the same socket. With A0 to A2 tied to ground, an AT24C02 answers at the same address as a 24LC02B, so the same code works for both.

1. Wire VCC, ground, SDA and SCL, with 2.2 kΩ from each line to VCC, and A0 to A2 and WP to ground.
2. Run an I2C scanner. The chip should show up at 0x50.
3. Write one byte, poll for the acknowledge, then read it back.
4. Tie WP to VCC and write a different value. The read-back should still show the first one.

The logic levels lesson explains the 0.7 VCC and 0.3 VCC input thresholds in the table above.

Buy the Microchip 24LC02B for a new board, tie A0 to A2 and WP to ground, and fit 2.2 kΩ pull-ups on a short 5 V or 3.3 V bus. Write in 8-byte pages on page boundaries, poll for the acknowledge, and save only when a value changes.

Shops list the 24C02 in DIP-8 and SOIC-8, often without a maker named. If the code has to work with more than one chip on the bus, buy a branded AT24C02 or check the A pins with the bench test first. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.